High-strength steel sheets having a tensile strength of 590 MPa or more have recently been applied as structural parts for automobiles in wider and wider applications with growing needs to provide both better fuel efficiency and satisfactory crashworthiness of automobiles. The high-strength steel sheets, however, have larger variations in mechanical properties such as yield strength, tensile strength, and work hardening coefficient than those of mild steels and thereby have disadvantages as follows. When the steel sheets are subjected to press forming, the variations cause a variation in springback and cause the resulting press-formed articles to fail to have satisfactory dimensional accuracy surely. In addition, the steel sheets should be designed to have a somewhat higher average strength so as to ensure required strengths of the press-formed articles even when they have a variation in strength. This leads to a shorter life of a press forming tool.
To solve the disadvantages, various efforts have been made to reduce variations in mechanical properties of high-strength steel sheets. The variations in mechanical properties of the high-strength steel sheets may be attributed to fluctuations in chemical composition and in manufacturing conditions. Based on this, proposals as follows have been made to reduce variations in mechanical properties.
Conventional Technology 1
Typically, Patent Literature (PTL) 1 discloses a technique of reducing variations in mechanical properties. The technique relates to a steel sheet and a manufacturing method thereof. The steel sheet has a dual phase structure of ferrite and martensite, where A as specified by expression: A=Si+9×Al meets a condition expressed as: 6.0≦A≦20.0. The manufacturing method of the steel sheet performs a recrystallization annealing-tempering treatment by holding the work at a temperature of Ac1 to Ac3 for 10 seconds or longer, slowly cooling the work from 500° C. down to 750° C. at a cooling rate of 20° C./s or less; thereafter rapidly cooling the work down to 100° C. or lower at a cooling rate of 100° C./s or more; and tempering the work at a temperature of 300° C. to 500° C. This allows the steel sheet to have a higher A3 point and thereby allows the dual phase structure to have better stability even when the rapid cooling start temperature, i.e., the slow cooling end-point temperature fluctuates.
Conventional Technology 2
PTL 2 discloses a technique for reducing variations in strength of a steel sheet. According to the technique, the variation reduction is performed by previously determining how the tensile strength of a steel sheet varies depending on the thickness, carbon content, phosphorus content, quench start temperature, quench stop temperature, and post-quenching tempering temperature; calculating the quench start temperature according to a target tensile strength in consideration of the thickness, carbon content, phosphorus content, quench stop temperature and post-quenching tempering temperature of the steel sheet to be manufactured; and starting quenching at the determined quench start temperature.
Conventional Technology 3
PTL 3 discloses a technique for improving (reducing) variations in elongation properties in a transverse direction of a steel sheet. The technique relates to a steel sheet having a microstructure including 3% or more of retained austenite, and a manufacturing method thereof. According to the technique, the variation reduction is achieved by an annealing treatment after cold rolling of a hot-rolled steel sheet. The annealing treatment is performed by soaking the work at a temperature of higher than 800° C. to lower than Ac3 point for a time of 30 seconds to 5 minutes; primarily cooling the work down to a temperature range of 450° C. to 550° C.; subsequently secondarily cooling the work down to a temperature of 450° C. to 400° C. at a cooling rate lower than the primary cooling rate; and further holding the work in a temperature range of 450° C. to 400° C. for one minute or longer.
The conventional technology 1 reduces microstructure fraction variations due to annealing temperature fluctuations by increasing the Al content to elevate the Ac3 point, whereby widening the dual-phase temperature range of Ac1 to Ac3, and reducing the temperature dependency of the steel in the dual-phase temperature range. In contrast, the present invention reduces variations in mechanical properties due to microstructure fraction variations by allowing fine cementite particles to disperse in a considerable number in ferrite grains to invite precipitation hardening and to increase ferrite hardness and by decreasing the carbon content in a hard secondary phase to reduce the hardness of the secondary phase, and thereby reducing the difference in hardness between the respective microstructures. The conventional technology 1 therefore fails to indicate the technical idea of the present invention. In addition, the conventional technology 1 has to increase the Al content and disadvantageously suffers from increased production cost of the steel sheet.
The conventional technology 2 changes the quench temperature according to the change in chemical composition and fails to reduce variations in elongation and stretch flangeability due to coil-to-coil fluctuations in microstructure fractions, although it can reduce variations in strength.
The conventional technology 3 fails to indicate variation reduction in stretch flangeability, although it refers to variation reduction in elongation.